Google Adsense have now rolled out a beta solution to responsive advertising. It’s not a complete solution, you will still need to specify pixels widths and be within their standard advertising sizes and it will only respond to the page load viewport. A good start.
The new responsive ad units allow you to support a wide range of devices by working with your responsive design web pages. You can now dynamically specify the size of the ad that will be served, adapting it to fit the way your site renders a page on a particular device.An excerpt from The responsive web + AdSense